August 21, 2012

Continents Resist Constant Movement Of The Earth’s Plates

Over the last 50 million years, the Caribbean islands have been pushed east, driven by the movement of the Earth's viscous mantel against the more rooted South American continent.

A new study by University of Southern California (USC) geophysicists, published in Nature Geoscience, gives us a better understanding of how continents resist the constant movement of the Earth's plates and what effect the continental plates have in reshaping the surface of the Earth.

"Studying the deep earth interior provides insights into how the Earth has evolved into its present form," said Meghan S. Miller, assistant professor of earth sciences in the USC Dornsife College of Letters, Arts and Sciences, and lead author of the paper. "We're interested in plate tectonics, and the southeastern Caribbean is interesting because it's right near a complex plate boundary."

Miller and her colleague, Thorsten W. Becker, studied the margin between the Caribbean plate and the South American plate ringed by Haiti, the Dominican Republic, Puerto Rico, and many smaller islands including Barbados and St. Lucia.

When it comes to the Earth, however, the First Law of Ecology applies. Everything really is connected. So to study the motion of the South American continent and the Caribbean plate, the researchers had to model the entire planet. In fact, they had to create 176 models that were so large it took several weeks to compute even at the USC High Performance Computing Center. That is an enormous amount of data, because the USC Center is the 7th fastest supercomputer cluster in the United States.

Most people dread even the idea of an earthquake, but not this research team. Earthquakes are a necessary source of data providing seismic waves that can be tracked all over the world to provide and image of the Earth's deep interior like a CAT scan.

Earthquake waves move slower or faster depending on the composition of the rock they are moving through, and how the crystal within the rocks align.

"If you can, you want to solve the whole system and then zoom in," Becker said. "What's cool about this paper is that we didn't just run one or two models. We ran a lot, and it allowed us to explore different possibilities for how mantle flow might work."

By reconstructing the movement of the Earth's mantle to a depth of almost 3,000 kilometers, Miller and Becker upend previous hypotheses of the seismic activity beneath the Caribbean Sea and provide an important new look at the unique tectonic interactions that cause the Caribbean plate to tear away from South America.

In particular, they point to a part of the South American plate — known as the "cratonic keel" — that is roughly three times thicker than normal lithosphere and much stronger than typical mantle. The keel deflects and channels mantle flow, and provides an important snapshot of the strength of the continents compared to the rest of the Earth's outer layers.

"Oceanic plates are relatively simple, but if we want to understand how the Earth works as a system — and how faults evolved and where the flow is going over millions of years — we also have to understand continental plates," Becker said.

In the southeastern Caribbean, the interaction of the subducted plate beneath the Antilles island arc with the stronger continental keel has created the El Pilar-San Sebastian Fault, and the researchers believe a similar series of interactions may have formed the San Andreas Fault.

"We're studying the Caribbean, but our models are run for the entire globe," Miller said. "We can look at similar features in Japan, Southern California and the Mediterranean, anywhere we have instruments to record earthquakes."